This invention relates to a method of switching between redundant routes of a communication system not having redundant transmission lines. More particularly, the invention relates to a method of switching between redundant routes of a communication system in which terminal stations having a redundant structure are connected by two transmission lines, namely uplink and downlink transmission lines, not having a redundant structure.
In an optical transmission system, a 1+1 line switching point-to-point system is constructed by adding a switch controller to a 1+1 configuration having one working line and one protection line, or a 1:N line switching point-to-point system is constructed by adding a switch controller to a 1:N configuration having N working lines and one protection line. When a working line fails, a changeover is effected to a protection line so that communication can continue.
The sending and receiving of information relating to the switching of optical signal lines is stipulated by the international standard (SONET) regarding SDH (Synchronous Digital Hierarchy). It is so arranged that K1/K2 bytes are used in overhead bytes OHB.
FIG. 10A is a diagram useful in describing a SONET STS-3 (OC-3) frame format. One frame is composed of 9.times.270 bytes. The first 9.times.9 bytes constitute section overhead (SOH), and the remaining bytes constitute path overhead (POH) and payload (PL). The section overhead SOH is the part of the frame that sends information (a frame synchronizing signal) representing the beginning of the frame, information specific to the transmission line (such as information which checks for error at the time of transmission and information for maintenance of the network), and a pointer which indicates the position of the path overhead POH. Further, path overhead POH is the part of the frame that sends end-to-end monitoring information in the network. The payload PL sends information at 150 Mbps.
The section overhead SOH is composed of repeater section overhead of 3.times.9 bytes, a pointer of 1.times.9 bytes and multiplex section overhead of 5.times.9 bytes. As shown in FIG. 10B, the repeater section overhead has bytes A1.about.A2, C1, B1, E1, F1, D1 .about.D3, and the multiplex section overhead has bytes B2, K1.about.K2, D4 D12, S1, Z1.about.Z2. The repeater section overhead and the multiplex section overhead each have a large number of undefined bytes the use of which is left to the particular communications concern.
The K1 byte among the overhead bytes is used mainly to request switching and designates the level of the switch request and the switched line. The K2 byte is used mainly to respond to the K1 byte and is employed also to represent system architecture, switching mode and AIS (Alarm Indication Signal)/FERF (Far End Receive Failure). Switching requests include, in addition to a request for switching at the time of signal failure, a switching request based upon forced switching and manual switching. FIGS. 11 and 12 illustrate the bits of the K1/K2 bytes stipulated by the SONET standard, as well as the meanings thereof.
K1 Byte
The first four bits b1.about.b4 of the K1 byte represent the switching request, and the remaining four bits b5.about.b8 represent the switched line and are capable of specifying a maximum of 14 transmission lines. "Lockout of Protection" is a switch request that prohibits switching to a protection transmission line. "Forced Switching" is a request for switching of an artificially designated transmission line. If a switch has been made, a switch will not be made to another a line even if a fault has occurred there. "SF" (Signal Failure) is a switching request for when a transmission line loses it signal. This request has two priorities, namely high and low. "SD" (Signal Degradation) is a switch request based upon signal degradation of a transmission line and has two priorities, namely high and low. The SF switching request has a higher priority than that of the SD switching request. "Manual Switching" is an artificial switching request. If a fault occurs somewhere, priority is given to switching of this location. "Wait to Restore" is a request wherein, even if a switch-back request is issued following restoration of a faulty line, switch-back is performed upon elapse of a predetermined period of time. "Exerciser" performs an actual switching to self-diagnose whether switching will be performed normally. "No Request" is sent when the situation is normal or when bridging is canceled.
K2 Byte
The first four bits b1.about.b4 of the K2 byte specify a transmission line number and are nulled (0000) in a case where the bits b5.about.b8 of a received K1 byte are null. In other cases these bits represent the number of the transmission line to which a changeover has been made. The b5 bit indicates the network configuration; "1" indicates a 1+1 system and "0" a 1:N system. The b6.about.b8 bits indicate the category of the switching mode, the specifics of the fault, etc. There are two types of switching modes, namely a unidirectional mode, in which only a signal in one direction is changed over, and a bidirectional mode, in which signals in both directions are changed over simultaneously.
Switching Sequence Using K1, K2 Bytes
In the case of the unidirectional mode, the K1 byte (switching request) is sent to a station A if a station B detects SF, as shown in FIG. 13A. The station A performs bridging control in regard to the line specified by the K1 byte (switching request) that has been received. Bridging control is control for sending identical signals to both working and protection lines. After performing bridging control, the station A transmits the K2 byte (switching response), which is in response to the received K1 byte, to the opposing station (station B). Upon receiving the K2 byte, the station B performs switching control. Switching control is control for switching a designated line signal in the receiving direction to a protection line.
In the case of the bidirectional mode, the K1 byte (switching request) is sent to station A if station B detects SF, as shown in FIG. 13B. Station A performs bridging control in regard to the line specified by the K1 byte (switching request) that has been received, sends back the K2 byte (switching response) in the same manner as in the unidirectional mode and, at the same time, sends the K1 byte designating "reverse request" (RR). Upon receiving RR, station B performs switching control and bridging control in regard to the line that was designated by the K1 byte sent by the B station itself and sends the K2 byte (switching response) to the opposing station (station A). Upon receiving the K2 byte, station A performs switching control.
FIG. 14 is a diagram useful in describing the details of switching based upon line protection using the K1 and K2 bytes. Shown in FIG. 14 are a terminal station (station A) 1, a terminal station (station B) 2 opposing the station A, a working transmission line 3 comprising a working uplink line 3a and a working downlink line 3b, and a protection transmission line 4 comprising a protection uplink line 4a and a protection downlink line 4b.
The terminal station (station A) 1 includes a multiplexer/demultiplexer 1a, working and protection transmitters (TX-W, TX-P) 1b, 1c, respectively, which send exactly identical signals, and working and protection receivers (RX-W, RX-P) 1d, 1e, respectively, which receive exactly identical signals. The terminal station (station B) 2 includes a multiplexer/demultiplexer 2a, working and protection transmitters (TXW, TX-P) 2b, 2c, respectively, which send exactly identical signals, and working and protection receivers (RX-W, RX-P) 2d, 2e, respectively, which receive exactly identical signals.
The working transmitter 1b of terminal station 1 is connected to the working receiver 2d of the terminal station 2 via the working uplink line 3a, and the protection transmitter 1c of terminal station 1 is connected to the protection receiver 2e of the terminal station 2 via the protection uplink line 4a. Similarly, the working transmitter 2b of terminal station 2 is connected to the working receiver 1d of the terminal station 1 via the working downlink line 3b, and the protection transmitter 2c of terminal station 2 is connected to the protection receiver 1e of the terminal station 1 via the protection downlink line 4b.
More specifically, in the system shown in FIG. 14, the terminal stations and transmission lines are both duplexed for redundancy so that when a fault develops in the working uplink line 3a, the system switches to the protection uplink line 4a. When a fault develops in the working downlink line 3b, the system switches to the protection downlink line 4b. For example, if the uplink line 3a develops a fault at the "x" mark and signal failure or signal degradation occurs, then the terminal station (station B) 2 detects SF or SD and sends the K1 byte (switching request) to the terminal station (station A) 1 via the protection line 4b. On the basis of the K1 byte which it has received, the station A performs bridging control, sends identical signals to both the working line 3a and protection line 4a and transmits the K2 byte (switching response), which is in response to the received K1 byte, to the opposing station (station B). Upon receiving the K2 byte, station B switches the line from the working line 3a to the protection line 4a by switching control. Thus, when an alarm is detected on a working or protection line, a changeover is made from the faulty line to the normal line.
When line redundancy is employed in a communication system covering a long distance and entailing a high transmission line cost (e.g., a submerged system required for international communications), providing a large number of lines and repeaters raises the cost of the communication system and is unrealistic. For this reason, use can be made of a communication system in which only the terminal equipment is provided with redundancy and not the lines.
FIG. 15 is an example of a communication system in which lines are not redundant. Components identical with those shown in FIG. 14 are designated by like reference characters. Shown in FIG. 15 are an additional unit 5 on the side of terminal A, an additional unit 6 on the side of terminal B, an uplink transmission line 7 and a downlink transmission line 8. The transmission lines 8 and 9 are not furnished with redundancy. The additional unit 5 has a switch 5a for selecting and sending the uplink transmission line 7 one of the signals that enter from the working and protection transmitters 1b, 1c, a hybrid circuit 5b for distributing a signal, which enters from the downlink transmission line 8 to the working and protection receivers 1d, 1e, and a controller 5c for designating the signal to be selected by the switch 5a. The additional unit 6 has a switch 6a for selecting and sending the downlink transmission line 8 one of the signals that enter from the working and protection transmitters 2b, 2c, a hybrid circuit 6b for distributing a signal, which enters from the uplink transmission line 7 to the working and protection receivers 2d, 2e, and a controller 6c for designating the signal to be selected by the switch 6a.
Assume that the signal of the working route is selected by the switch 5a in this communication and that this signal is being distributed to the working and protection receivers 2d, 2e by the hybrid circuit 6b. In the event that a fault develops at point x under these circumstances, the terminal station (station B) 2 detects Signal Failure SF or Signal Degradation SD in working and protection lines 3a', 4a' simultaneously. If SF or SD is sensed simultaneously in the working and protection lines, line switching is not performed according to the stipulations of the international standard since such switching would be meaningless. With the arrangement of FIG. 15, however, continuation of communication becomes possible if the line is switched from a working line 3a to a protection line 4a. This means that with a communication system not having line redundancy, it is necessary to arrange it so that communication can be continued by line switching even in a case where SF or SD is sensed simultaneously in the working and protection lines.
Progress has recently been made in communication techniques employing ultra-high-speed bit rates, communication techniques (optical amplification, code correction, etc.) using relay span extension means and optical wavelength multiplexed communication techniques, and various devices have been developed on the basis of these techniques. Such devices are placed between an SDH unit and a transmission line in an effort to reduce communication costs even if only marginally. However, there are instances where not all transmission line alarms can be detected in these devices, in which case it is necessary to deal with this by performing detection on the side of the SDH unit.
The communication system shown in FIG. 16 has been proposed to meet this requirement. This communication system has two 2.5-Gbps SDH units (SDH MUX A, SDH MUX C) 11, 12 and a 5-Gbps higher level unit (SLTE-A) 13 provided on the side of station A, and two 2.5-Gbps SDH units (SDH MUX B, SDH MUX D) 14, 15 and a 5-Gbps higher level unit (SLTE-B) 16 provided on the side of station B. The higher level units 13 and 16 are connected by uplink and downlink transmission lines 17, 18. The higher level unit 13 multiplexes the 2.5-Gbps signals from the SDH units (SDH MUX) 11, 12 to obtain a 5-Gbps signal, sends this signal to the transmission line 17, branches a 5-Gbps multiplexed signal from the transmission line 18 to the working and protection routes, demultiplexes each of these branched signals to a 2.5-Gbps signal and inputs the 2.5-Gbps signals to the SDH units (SDH MUX) 11, 12. The higher level unit 16 multiplexes the 2.5-Gbps signals from the SDH units (SDH MUX) 14, 15 to obtain a 5-Gbps signal, sends this signal to the transmission line 18, distributes a 5-Gbps multiplexed signal from the transmission line 17 to the working and protection routes, demultiplexes each of these signals to a 2.5-Gbps signal and inputs the resulting signals to the SDH units (SDH MUX) 14, 15.
The SDH units 11, 12 and 14, 15 have identical structures and each has the components indicated below. Specifically, each SDH unit (SDH MUX) includes:
(1) a working transmitter (WTX) 21; PA1 (2) a working receiver (WRX) 22; PA1 (3) a protection transmitter (PTX) 23; PA1 (4) a protection receiver (PRX) 24; PA1 (5) a multiplexer (MUX) 25 for multiplexing signals that enter from in lines #1.about.#11 and distributing the multiplexed signal to the working/protection transmitters 21, 23; PA1 (6) a demultiplexer (DMUX) 26 for demultiplexing one of the multiplexed signals that enter from the working/protection receivers 22, 24; PA1 (7) an alarm detector (ALM) 27 for detecting an alarm in the opposing station sent from the opposing station and outputting a line switching request; PA1 (8) a fault detector (ALM) 28 for detecting signal failure SF or signal degradation SD and designating transmission of an opposing station alarm; and PA1 (9) a controller (CONT) 29 for sending an opposing station alarm in response to an opposing station alarm transmission trigger. PA1 (1) a working multiplexer/demultiplexer (WMLDM) 31 which multiplexes 2.5-Gbps signals from the working transmitters 21 of the SDH (SDH MUX) units 11, 12 to obtain a 5-Gbps signal, sends this signal to the transmission line 17 via a switch, demultiplexes a 5-Gbps multiplexed signal that enters from the transmission line 18 via a hybrid circuit to 2.5-Gbps signals and inputs these signals to the working receivers 22 of the SDH units (SDH MUX) 11, 12; PA1 (2) a protection multiplexer/demultiplexer (PMLDM) 32 which multiplexes 2.5-Gbps signals from the protection transmitters 23 of the SDH (SDH MUX) units 11, 12 to obtain a 5-Gbps signal, sends this signal to the transmission line 17 via a switch, demultiplexes a 5-Gbps multiplexed signal that enters from the transmission line 18 via a hybrid circuit to 2.5-Gbps signals and inputs these signals to the protection receivers 24 of the SDH units (SDH MUX) 11, 12; PA1 (3) a switch (SW) 33 for selecting, and sending to the transmission line 17, one of the multiplexed signals that enters from the working multiplexer/demultiplexer (WMLDM) 31 and protection multiplexer/demultiplexer (PMLDM) 32; PA1 (4) a hybrid circuit (HYB) 34 for distributing the 5-Gbps multiplexed signal that enters from the transmission line 18 to the working and protection multiplexer/demultiplexers 31, 32; and PA1 (5) a switch controller (SW CONT) 35 for controlling the switch 33, in response to a line switching request specified by the alarm detector (DET) 27, to send a working or protection signal to the transmission line 17. PA1 (1) If a failure occurs at the location indicated at x, the signal on the line indicated by the dashed line is lost (SF) or degraded (SD). PA1 (2) As a result, the transmission line alarm (SF, SD) is detected simultaneously by the working and protection receivers 22, 24 in the SDH unit (SDH MUX B) 14. PA1 (3) In response to detection of the alarm (signal failure SF or signal degradation SD) simultaneously in both the working and protection routes, the fault detector (ALM) 28 generates the opposing station alarm transmission trigger. In the event that the alarm is detected in only one route, the opposing station alarm transmission trigger is not generated. PA1 (4) Upon receiving the opposing station alarm transmission trigger, the controller (CONT) 29 transmits the opposing station alarm to the SDH unit 11 along the path of the dot-and-dash line using the K2 byte of the overhead OHB. PA1 (5) The working and protection receivers 22, 24 of the SDH unit 11 detect the opposing station alarm sent from the SDH unit 14 and notify the alarm detector (DET) 27. PA1 (6) In response to detection of the opposing station alarm in either the working or protection route, the alarm detector (DET) 27 issues a switching request to the higher level unit 13. PA1 (7) Upon receiving the switching request, the higher level unit 13 instructs the switch 33 to switch lines. In response to this indication to switch, the switch 33 selects the signal from the protection multiplexer/demultiplexer 32 instead of the signal from the working multiplexer/demultiplexer 31 and sends the selected signal to the transmission line 17. As the result of these operations, communication is allowed to continue.
It should be noted that not all of the above-mentioned components are shown in the SDH units 11, 12 and 14, 15; only those components necessary for descriptive purposes are illustrated.
The higher level units (SDH MUX) 13, 16 have identical structures and each has the components indicated below. For example, the higher level unit (SDH MUX) 13 includes:
It should be noted that not all of the above-mentioned components are shown in the higher level units 13, 16; only those components necessary for descriptive purposes are illustrated.
Described next will be a line switching procedure in a case where a line failure occurs at point x when a working signal is being transmitted in the uplink direction via the switch 33 and the transmission line 17 along the path indicated by the dashed line and, similarly, when a working signal is being transmitted in the downlink direction via the switch 33 and the transmission line 18 along the path indicated by the dot-and-dash line. In this case, (a) the higher level unit (SLTE MUX A) 13 implements switching, (b) the SDH unit (SDH MUX B) detects the transmission line alarm and transmits the opposing station information, and (c) the SDH unit (SDH MUX A) 11 detects the alarm (opposing station alarm) from the opposing station and outputs the line switching request.
For example, if a failure occurs at point b between the multiplexer/demultiplexer 31 of the higher level 16 and the working receiver 22 of the SDH unit 14, the fault detector (ALM) 28 detects an alarm (signal failure SF or signal degradation SD) solely in the working route. In such case the fault detector (ALM) 28 so notifies the controller 29. As a result, the controller 29 controls a demultiplexer (not shown) so that the demultiplexer demultiplexes the signal from the protection receiver 24 instead of the signal from the working receiver 22 and sends the demultiplexed signals to the prescribed lines #1.about.#n.
Thus, with the arrangement shown in FIG. 16, line switching can be controlled even in a case where an alarm (signal failure SF or signal degradation SD) is detected simultaneously in both the working and protection routes of a communication system not having line redundancy.
However, a problem which arises is that the prior-art SDH unit, in contravention of the stipulation of the international standard, performs line switching in a case where an alarm is detected in working and protection routes simultaneously.
Further, when a switching request is issued, the prior art is such that switching is implemented in toggled fashion; whether the switching request is from the working side to the protection side or from the protection side to the working side cannot be determined. Consequently, if switching occurs frequently, a problem which arises in the prior art is confusion and an inability to perform switching correctly.
In an effort to solve these problems, the insertion of an alarm on the side of the opposing station is canceled after the completion of a switching operation, guard time is provided for the period of time required for the canceled signal to return to the station that issued it, and completion of switching is verified. However, communication is interrupted for the duration of switching verification.
Further, line switching in the event of failures in the transmission lines 17 and 18 is meaningless and unnecessary. Nevertheless, needless line switching is performed in such case according to the prior art.